rsyslog: add logrotate for all-machines.log
rsyslog: logrotate comments and docs
rsyslog: logrotate tests
rsyslog: check for logrotate files, updated comments
rsyslog: only render templates once, improve file check test
rsyslog: comments for exported functions
rsyslog: updated documentation, simplified logrotate file rendering.
Merge pull request #511 from wwitzel3/023-rsyslog-logrotate

rsyslog: configure and setup log rotation for all-machines.log

Implements sized based rotation for the all-machines.log.

See: https://bugs.launchpad.net/juju-core/+bug/1078213